md: make bitmap file support optional

The support for write intent bitmaps in files on an external files in md
is a hot mess that abuses ->bmap to map file offsets into physical device
objects, and also abuses buffer_heads in a creative way.

Make this code optional so that MD can be built into future kernels
without buffer_head support, and so that we can eventually deprecate it.

Note this does not affect the internal bitmap support, which has none of
the problems.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230615064840.629492-11-hch@lst.de

drivers/md/Kconfig

@@ -50,6 +50,16 @@ config MD_AUTODETECT
 
 	  If unsure, say Y.
 
+config MD_BITMAP_FILE
+	bool "MD bitmap file support"
+	default y
+	help
+	  If you say Y here, support for write intent bitmaps in files on an
+	  external file system is enabled.  This is an alternative to the internal
+	  bitmaps near the MD superblock, and very problematic code that abuses
+	  various kernel APIs and can only work with files on a file system not
+	  actually sitting on the MD device.
+
 config MD_LINEAR
 	tristate "Linear (append) mode (deprecated)"
 	depends on BLK_DEV_MD

drivers/md/md-bitmap.c

@@ -295,6 +295,7 @@ static void write_sb_page(struct bitmap *bitmap, unsigned long pg_index,
 
 static void md_bitmap_file_kick(struct bitmap *bitmap);
 
+#ifdef CONFIG_MD_BITMAP_FILE
 static void write_file_page(struct bitmap *bitmap, struct page *page, int wait)
 {
 	struct buffer_head *bh = page_buffers(page);
@@ -408,6 +409,20 @@ static int read_file_page(struct file *file, unsigned long index,
 		       ret);
 	return ret;
 }
+#else /* CONFIG_MD_BITMAP_FILE */
+static void write_file_page(struct bitmap *bitmap, struct page *page, int wait)
+{
+}
+static int read_file_page(struct file *file, unsigned long index,
+		struct bitmap *bitmap, unsigned long count, struct page *page)
+{
+	return -EIO;
+}
+static void free_buffers(struct page *page)
+{
+	put_page(page);
+}
+#endif /* CONFIG_MD_BITMAP_FILE */
 
 /*
  * bitmap file superblock operations

drivers/md/md.c

@@ -7043,6 +7043,13 @@ static int set_bitmap_file(struct mddev *mddev, int fd)
 
 		if (mddev->bitmap || mddev->bitmap_info.file)
 			return -EEXIST; /* cannot add when bitmap is present */
+
+		if (!IS_ENABLED(CONFIG_MD_BITMAP_FILE)) {
+			pr_warn("%s: bitmap files not supported by this kernel\n",
+				mdname(mddev));
+			return -EINVAL;
+		}
+
 		f = fget(fd);
 
 		if (f == NULL) {